The bones and connective tissue of an adult human spinal column consists of more than twenty discrete bones coupled sequentially to one another by a tri-joint complex which consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than twenty bones are anatomically categorized as being members of one of four classifications: cervical; thoracic; lumbar; or sacral.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the causes that can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art which achieve immobilization and/or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back that needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. With respect to the failure of the intervertebral disc, and the insertion of implants and/or height restorative devices, several methods and devices have been disclosed in the prior art.
More particularly, and with respect to the historical development of the present surgical methods and instrumentations, the description of the relevant medical techniques are now described. Failure of the intervertebral disc cartilage generally includes a loss of proper anatomical spacing between the end plates of the opposing vertebral bodies. This loss of height may simply destabilize the spine, or, in severe cases, it may cause considerable neurological impairment as the nerve roots are compressed by the converging lateral extensions of the bones (e.g., in the facet joint).
Restoring the appropriate height to the intervertebral space is the first step in the surgical strategy for correcting this condition. Once this is achieved, one class of surgical implantation procedures involves positioning a device into the intervening space. This may be done through a posterior approach, a lateral approach, or an anterior approach. Various implant devices for this purpose include femoral ring allograft, cylindrical metallic devices (e.g., cages), and metal mesh structures that may be filled with suitable bone graft materials. Some of these implant devices are only suitable for one direction of approach to the spine. All of these devices, however, are provided with the intention that the adjacent bones will, once restored to their appropriate separation, then grow together across the space and fuse together (or at least fuse into the device implanted between the bones).
Most recently, the development of non-fusion implant devices, which purport to permit continued natural movement in the tri-joint complex, have provided great promise. The instrumentation and methods for the implantation of these non-fusion devices, as well as the implantation of the fusion devices catalogued previously, therefore should integrate the functions of restoring proper anatomical spacing and easing insertion of the selected device into the formed volume.
To these ends, several instruments for such implantation have been described in the prior art. These include U.S. Pat. No. 6,159,215 to Urbahns, et al. (“Urbahns”), U.S. Pat. No. 6,042,582 to Ray (“Ray”), and U.S. Pat. No. 5,431,658 to Moskovich (“Moskovich”). More particularly, Ray describes a device and method of implantation for use specifically with cylindrical cage devices that are inserted such that the axis of the implant device is perpendicular to the axis of the spine. The reference teaches the use of a series of similarly shaped plugs to be inserted posteriorly between the collapsed bones, for the purposes of separating the adjacent bones, followed by the cutting of the endplates to receive the threaded implant. Urbahns teaches the use of intervertebral space measuring tools and a spacer insertion device for facilitating the implantation of an intervertebral spacer (in this reference, the spacer implant is a tubular metal mesh structure which is coaxial with the patient's spine). The measuring device described and shown in FIG. 4 in Urbahns includes a thin, elongate rod having a fixed cylindrical end having a constant and known thickness. Insertion of this measuring tool into the intervertebral space provides the physician with an approximate understanding of the size of the implant to be inserted. This measurement defines the appropriate cutting of the patient's bone to create the desired, and necessary, space to receive the metal mesh. The measuring tool is, however, not used to distract the space.
Urbahn describes a distraction provided in conjunction with the spacer insertion instrument shown in FIGS. 13-16 of the reference. This instrument, which is more fully described and shown in FIG. 4 in Moskovich, includes a pair of flat elongate guide surfaces that are hinged at an elbow joint at the distal ends of the surfaces. The distal joint is designed to extend out of the planes defined by the longitudinal axes of the two guides. The proximal ends of the surfaces are to be placed between the collapsed bones. By virtue of the elbow joint, the surfaces are angled substantially when the metal mesh structure, or test member, is placed between the surfaces. The metal mesh (or the test member) is then hammered down the guide surfaces, prying the vertebral bodies apart.
Moskovich is generally directed to a threaded insertion device for final placement of the femoral ring (not a metal mesh structure) into the intervertebral space. A threaded shaft, having a distal ram portion and an intermediate nut, is coupled to the guide surfaces via stud-groove interfaces that engage studs on the intermediate nut and corresponding grooves on the elongate guide surfaces. The ram portion seats against the femoral ring and causes it to move relative to the guides. The space into which the femoral ring is to be inserted (as above with the metal mesh implant) must be cut to the appropriate size to receive the graft. Initially, the surgeon rotateably advances the graft into the space. Subsequent to proper placement of the graft (i.e., when the graft jams into the pre-cut receiving space), continued rotation of the shaft causes the distraction surfaces to be removed by motion of the guides relative to the shaft (the intermediate nut engages the guides and pulls them free of the vertebral bones). Failure to properly cut the space, or structural failure of the graft and/or bone material, will prevent removal of the guides, and further rotation of the shaft will drive the allograft further than clinically desired (risking paralysis and/or damage to surrounding vessels).
Therefore, there is a need for a vertebral implant insertion device that does not require the space into which the implant is to be inserted to be cut to a specific size or shape so that the implant must jam into the space to effect the removal of the guides. There is also a need for an insertion device that does not increase the risk of structural failure of the implant during the insertion and does not depend on the structural stability of the implant to effect the insertion.